A sentence with double negative [closed]

Usually when people talk about a "double negative" they mean a construction like

I didn't see no-one.

where a negative-polarity item like no, none, nothing is in the scope of an explicitly negative verb. They are of note because standard varieties of English don't allow them (and people try to rationalise the objection by the claim that the above "means" I saw someone).

That's not the case here, as both negatives are not (hadn't = had not), and they're in different clauses: if Billy hadn't and not to.

So though you could call this a double negative, it is actually a straightforward application of the two negative senses.

For the other question, as others have said would have does imply that he didn't.

[does] "would have gone"... imply that he didn't go after all?

Basically, yes.

It's a double negative, but here the doubled negative does not cancel out, but rather strengthens, It intensifies the negation, meaning it's a negative concord (NC) which was common in Middle English; some scholars say it's gone from the language; others say it's still around but less commonly that in other (eg Romance) languages.

It is largely assumed that the loss of NC was the outcome of prescriptive views on language use...

I've read three scholarly parers and am none the wiser (it's practically a foreign language to me) as to why. But it makes sense to me, and does not seem to me to be nonstandard usage. It reads clearly, and without ambiguity.

Particularly striking is the asymmetry between variation in the first and the second negative elements. Only five negators act as the first element in a clause containing a negative concord structure. Of these, n’t/not is used in…88% or almost nine out of ten of all cases.

For example, in response to "How can you admire a man like that?" one can answer:

I can’t not admire him.

The way this is phrased, it intensifies the admiration.

The context is

Kilgore Trout was shadowing him, keen to know what Billy had suspected or seen...

Billy fled upstairs in his nice white home.

Trout would have come upstairs if Billy hadn’t told him not to. Then Billy went into the bathroom, which was dark. He closed and locked the door. He left it dark, and gradually became aware that he was not alone. His son was there.

(Billy is being thrown around backwards and forwards through time. Trout is a guy who wants to prove time warps exist, and he suspects Billy is experiencing them. So, Trout was following Billy around hoping to learn something.

Billy wanted to be alone. He knew that if he did not ask Trout not to come upstairs, Trout would have followed him up, because Trout was shadowing Billy for evidence is the existence of time warps. So Billy asks Trout not to come up. As a result, Trout did not follow him.)

It's a double negative, but here the doubled negative intensifies the negation, meaning it's a negative concord.

I've read three scholarly parers and am none the wiser (it's practically a foreign language to me) as to why. But it makes sense to me, and does not seem to be nonstandard usage.

If the following makes any sense to you, maybe you can explain it.

_{from Negative Concord in Standard English and the syntax-morphology interface by Susagna Tubau
Aspects of English Negation, Yoko Iyeiri}